The present invention relates to a method and a circuit according to the preamble of the independent claims.
In particular the present invention relates to a method for adapting a first reference value. The first reference value serves for generating a first bit stream from an input signal by an amplitude adapting unit. The input signal comprises a first signal and a second signal. The first signal has a first signal amplitude and the second signal a second signal amplitude. The first signal amplitude and the second signal amplitude form a mixed amplitude. A first non-linear component demodulates the input signal and outputs the demodulated input signal as a mixed signal. On the basis of the first reference value, the amplitude adapting unit outputs a first bit stream from the mixed signal or the demodulated input signal.
Further, the present invention relates to in particular a circuit for adapting a first reference value for generating a first bit stream from an input signal. The input signal comprises a first signal and a second signal, wherein the first signal has a first signal amplitude and the second signal a second signal amplitude. The first signal amplitude forms a mixed amplitude with the second signal amplitude. The circuit has a first non-linear component and a first amplitude adapting unit. The first non-linear component is configured to demodulate the input signal and to output a demodulated input signal. The first amplitude adapting unit is configured to generate the first bit stream from the demodulated input signal on the basis of the first reference value.
Various methods are known for receiving input signals and demodulating signals, in particular by a contactless security module. A receiving unit receives a first signal from a transmitting/receiving device. The received first signal is usually modified by way of an impedance transducer, in particular an apparatus and/or unit for multiplying a voltage. With the impedance transducer, an amplitude, in particular a voltage amplitude, of the received first signal is increased. If the first signal is an amplitude-modulated signal, the first signal is demodulated with the help of the impedance transducer, which comprises at least one non-linear component. The impedance transducer is usually a voltage multiplier.
For generating an output bit stream from the first signal, it is processed by means of an amplitude adapting unit. The amplitude adapting unit creates a bit stream of the first signal from the modified first signal. The amplitude adapting unit usually comprises a comparator with which the demodulated first signal is converted into a binary signal. The demodulated first signal is furthermore smoothed in a second current path by means of a smoothing unit and therefrom a reference value is formed for the comparator. The reference value, which is preferably smaller than the peak amplitude of the demodulated first signal at the input of the amplitude adapting unit, holds as a reference value relative to the demodulated first signal for generating a first output bit stream.
Due to a steady rise of wireless communication, the number of the signals which are modulated on different carrier frequencies is rising. Besides the first signal, the contactless security module often receives further signals, at least one second signal. Particularly if the second signal has a second carrier frequency, which lies close to the first carrier frequency of the first signal and the first carrier frequency and the second carrier frequency are therefore in the identical reception range of the security module, and moreover the second signal has an amplitude approximately of the order of magnitude of the first signal, the two carrier frequencies of the first signal and the second signal influence each other such that the security module cannot generate from the input signal an unambiguous output bit stream relating to the first signal. As a rule, only an interference value is generated. A communication between transmission/receiving device and the contactless security module is no longer ensured.
For example, the frequency ranges of GSM and UHF-RFID not only lie in immediate adjacency, but they partly overlap. For example, a UHF-RFID reading device transmits in the frequency range of 865 MHz and a GSM mobile phone in the frequency range of 880-915 MHz. Due to a simple design, a UHF-RFID transponder is ready to receive on a broad band. The reception range of the UHF-RFID transponder is usually limited only by the antenna. The UHF-RFID transponder can therefore receive UHF-RFID signals as well as GSM signals of a mobile phone in the vicinity. The two signals superimpose each other. The UHF-RFID transponder of the prior art cannot separate and decode the UHF-RFID signals. A communication between the UHF-RFID transponder and a UHF-RFID transmitting/receiving device is not possible as soon as the amplitude of the GSM signal exceeds a limit amplitude in proportion to the UHF-RFID signal.